A valve system of this general type is described in U.S. Pat. No. 3,399,510 which relates to a dry air plant in which a valve system lying between the outlet of a compressor and the inlet of an adsorption filter contains an oil-water trap serving to remove liquid condensation products from a supersaturated stream of compressed air and to collect them in a water chamber during the operation of the compressor. In the known valve system, a tap opening for discharging the collected liquid is opened by means of a solenoid valve which in turn is controlled by a pressure sensitive switch which responds as soon as a predetermined pressure has been reached in a compressed air tank to stop the compressor and simultaneously energize the solenoid valve. Upon opening of the tap opening, the collected condensate can flow out of the water chamber, followed then by the air which, with the compressor stationary, is conducted in a reverse direction through the absorption filter in order to regenerate the latter.
It has been found that the known valve system described above does not satisfy all requirements since it needs a separate control which leads to difficulties, particularly in case of export, inasmuch as frequently different power line voltages and/or frequencies are present at the place of reception and furthermore different electrical safety regulations apply than in the country of manufacture. This has the result that in order to place the equipment into operation, conversion work is frequently necessary which is in all cases expensive and can furthermore, if it is not carried out properly, lead to technical difficulties and to complaints.